The invention relates to pressure monitoring devices and techniques, and in particular to a method and apparatus for monitoring the intracranial pressure of the cerebrospinal fluid of mammals without the necessity for surgical or other implantation within the cisterna magna, the lateral cerebral ventricles or the lumbar subarachnoid space of instruments such as needles, catheters and the like. The present invention also enables the monitoring of intracranial pressure without the implantation of sensors, meters, capsules or other transducer type devices within or beneath the surface of the skull.
The ability to monitor the intracranial pressure of an injured or diseased patient has long been of significant diagnostic and postoperative importance in the medical profession, particularly with respect to patients suffering diseases known or suspected of having an affect upon the pressure of the subarachnoidal fluid adjacent the brain.
For example, intracranial pressure monitoring is particularly desirable for hydrocephalic individuals, for individuals who have undergone neurosurgery, and even for those subject to or suspected of experiencing brain swelling, edema, obstruction of cerebrospinal fluid passages, tumors, hemorrhages, infections and the like. Any of these situations can result in sufficient interference with the normal functioning of the regulatory system for controlling pressure of the cerebrospinal fluid (i.e., the rate of cerebrospinal fluid formation and the resistance to absorption through the arachnoidal villi), to increase the chances that intracranial pressure will rise to dangerous levels.
One known technique for measuring the pressure of the cerebrospinal fluid requires the subject to lie precisely horizontally on its side in order to equalize the pressures in the spinal column and the cranial vault. A hollow needle is then inserted into the lumbar spinal canal below the lower end of the cord, and is connected to a glass tube. The spinal fluid passes through the needle out of the spinal canal and rises in the tube to a level corresponding to its internal pressure. Suitable calibration of the tube enables a convenient measure of the fluid pressure, preferably in mmHg.
This technique however is not favored by the medical profession because of the likelihood that in the event of relatively high cerebrospinal fluid pressures, dangerously excessive fluid losses are likely (and have been known) to occur. Moreover, since the cerebrospinal fluid does not clot, additional fluid losses will occur until the punctured dura heals, generally over a period of about one week. In addition, the invasive use of a hollow needle generates considerable patient discomfort and involves a persistent danger of infection. A further disadvantage of this technique, as well as a related technique involving the introduction of a catheter into the ventricular spaces of the brain, is that it is not suitable for monitoring intracranial pressure over prolonged periods of time.
Accordingly, a variety of techniques for measuring intracranial pressure have evolved which avoid the penetration of the dura mater membrane enclosing the central nervous system. These prior methods have commonly involved implantation within the head of the patient of a pressure transducer having wires or tubes which pass outwardly through the scalp and skull for connection to a recording device.
For example, in U.S. Pat. No. 3,877,137 there is disclosed an hydraulic pressure sensor in the form of a compressible bulb or bladder filled with displaceable fluid and adapted for implantation between the skull and the brain of a patient. A pressure sensor which generates optical signals has been suggested (U.S. Pat. No. 3,686,958) for placement within the skull and which is connected to a suitable external interpretive device by way of optical fibers.
In addition, various pressure sensors (U.S. Pat. Nos. 3,757,770 and 4,062,354) have been devised which transmit electrical signals or radiate representative electromagnetic energy to an external receiving or recording apparatus. For example, U.S. Pat. No. 4,003,141 discloses a pressure sensor to be mounted within the skull and which is connected electrically to an external recording mechanism, thereby subjecting the patient to the attendant hazards of electrical shock. Other pressure sensors operating for example on the basis of a discrete mass of radioactive material (U.S. Pat. No. 3,977,391) have also been proposed heretofore. The high medical risk attendant to the use of such a device is self-evident.
Each of these prior devices and techniques has suffered from the disadvantage of requiring placement of the transducer within the cranium of the patient. Implantation and subsequent removal of the pressure transducer exposes the patient to certain surgical risks including a persistent risk of infection. Moreover, these prior systems require periodic adjustment to compensate for changes in the position of a patient and, in some cases, are known to have a tendency to cause leakage or blockage within the skull and therefore affect the measurements being derived.
The result has been a general reluctance on the part of physicians to make use of this diagnostic tool except under the most severe circumstances so that many patients suffering head injuries, for example, have not had the benefit of intracranial pressure monitoring. A dangerous rise in intracranial pressure (which may result, for example, from a small internal hemorrhage) has therefore often gone undiagnosed until clinical symptoms have developed. At this point, it is often too late to provide effective remedial aid (such as through the use of appropriate drugs) short of drastic surgical intervention.
These and other disadvantages of the prior techniques and systems for measuring intracranial pressure have been obviated by the present invention which provides a completely noninvasive apparatus and technique for deriving information relative to the intracranial pressure. The present technique is based essentially upon the heretofore unrealized relationship between the presure of blood in the jugular vein outside of the head and the pressure exerted by the cerebrospinal fluid within the dura membrane of the head. It has been discovered that the intracranial pressure has a detectable effect upon the pressure of blood within the network of vessels throughout the cranial cavity, and particularly within those venous blood vessels lying within the subarachnoid and perivascular spaces. This effect is transmitted to and is reflected by the pressure of blood within the jugular vein in the neck. Since pressure within the jugular is related to the rate of flow of blood therein, a noninvasive technique may be utilized to detect the blood flow through that vein so as to yield determinable information concerning the intracranial pressure of the cerebrospinal fluid.
It has been discovered that the effects of the intracranial pressure on the jugular pressure are most readily determined during a relatively short interval of time following total occlusion of the jugular so as to cause an interruption in the flow of blood through the vein. During that short interval (on the order of five seconds) the blood flow approaches zero and the blood pressure increases markedly at a rate of change which varies with the intracranial pressure. Accordingly, a measurement of the change in jugular pressure or flow with time following occlusion will provide an accurate indication of the intracranial pressure during that same time interval.
Blood flow may be measured noninvasively by inducing and detecting a transcutaneous electrical impulse representative of blood flow within the jugular vein. This may be accomplished through the placement of suitable transducer electrodes on the surface of the skin of the neck of a patient on opposite sides of the jugular vein. A narrow width magnetic flux of low intensity is then passed through the vein between the electrodes at a direction perpendicular to the flow of blood to induce an electrical impulse to appear at the surface of the skin to be detected by the transducer electrodes. Following occlusion of the jugular vein at a point downstream of electrodes, the resulting electrical signals (representative of the rate of change of flow as the flow approaches zero) are transmitted to a receiving and control circuit by which they are filtered, amplified and manipulated or processed computationally to a value representative of the intracranial pressure. The resulting value of intracranial pressure may be displayed digitally or otherwise as desired for easy reference by an attending physician or other medical personnel.